Computers have used rotating magnetic media for mass storage of data, programs and information. Though widely used and commonly accepted, such hard disk drives suffer from a variety of deficiencies. Because of the rotation of the disk, there is an inherent latency in extracting information from a hard disk drive.
Other problems are especially dramatic in portable computers. In particular, hard disks are unable to withstand many of the kinds of physical shock that a portable computer will likely sustain. Further, the motor for rotating the disk consumes significant amounts of power decreasing the battery life for portable computers.
Solid state memory is an ideal choice for replacing a hard disk drive for mass storage because it can resolve the problems cited above. Potential solutions have been proposed for replacing a hard disk drive with a semiconductor memory. For such a system to be truly useful, the memory must be non-volatile and alterable. The inventors have determined that flash memory is preferred for such a replacement. It should be noted that E.sup.2 PROM is also suitable as a replacement for a hard disk drive.
Flash memory is a single transistor memory cell which is programmable through hot electron injection and erasable through Fowler-Nordheim tunneling. The programming and erasing of such a memory cell requires current to pass through the dielectric surrounding a floating gate electrode. Because of this, such types of memory have a finite number of erase-write cycles. Eventually, the dielectric will fail. Manufacturers of flash cell devices specify the limit for the number erase-write cycles as between 10,000 and 100,000. Accordingly, unlike rotating magnetic media, a flash memory mass storage device does not have an indefinite lifetime.
Another requirement for a semiconductor mass storage device to be successful is that its use in lieu of a rotating media hard disk mass storage device be transparent to the system designer and the user. In other words, the designer of a computer incorporating such a semiconductor mass storage device could simply remove the hard disk and replace it with a semiconductor mass storage. All presently available commercial software should operate on a system employing such a semiconductor hard disk without the necessity of any modification.
SunDisk proposed an architecture for a semiconductor mass storage using flash memory at the Silicon Valley PC Design Conference Jul. 9, 1991. That mass storage system included read-write block sizes of 512 Bytes (or multiples thereof) just like IBM PC compatible hard disk sector sizes. (IBM PC is a trademark of IBM Corporation.) During an erase cycle, an entire block is first fully programmed and then erased.
As in conventional hard disks, it appears in the SunDisk architecture that there is an erase-before-write cycle each time data is changed in the mass storage. Thus, if a program or data block is to be changed, the data is written to RAM and appropriately changed, the flash block is fully programmed, then erased and then reprogrammed to the new memory condition. Unlike a hard disk device, in a flash memory device an erase cycle is slow which can significantly reduce the performance of a system utilizing flash memory as its mass storage.
Though such an architecture provides a workable semiconductor mass storage, there are several inefficiencies. First of all, each time a memory block is changed, there is a delay to the entire system due to the necessary erase-before-write cycle before reprogramming the altered information back into the block. The overhead associated with erase-before-write cycles is costly in terms of system performance.
Secondly, hard disk users typically store both information which is rarely changed and information which is frequently changed. For example, a commercial spread sheet or word processing software programs stored on a user's system are rarely, if ever, changed. However, the spread sheet data files or word processing documents are frequently changed. Thus, different sectors of a hard disk typically have dramatically different usage in terms of the number of times the information stored thereon is changed. While this disparity has no impact on a hard disk because of its insensitivity to data changes, in a flash memory device, this variance can cause sections of the mass storage to wear out and be unusable significantly sooner than other sections of the mass storage.